Sorry for replying so late.
1st thing you need is to host your image in some hosting website, i for exmaple use this one:
www.imgur.comAfter you upload the image you will need the BBCode from it, which you can find on the right side of the page, click it once and press Ctrl + C to copy.
2nd thing you need to do is simply go to your Forum Profile and on the Modify Porfile select: Customize Profile
Then scroll down and go to the Signature box, there you inser this code:
[center][img]*here you isnert the link of the image you just copied*[/img][/center]
Then just Save the changes and you are done.